给你这个曲面和这个曲线作为它的边。

Let's say that I gave you this curve bounding this surface.

youdao

当我在曲线上时，我是在曲面的边上的，所以一个是沿着曲面的边指，那个就是曲线的定向，另外一个就要指向曲面里面的那个方向。

And when I am on the curve I am on the boundary of the surface, so there is a direction along the surface that is the curve and the other one is pointing into the surface.

youdao

如果完全封闭它，那么这条曲线就不再是它的边界，因为曲面在这条曲线的两边都存在。

If I close it completely, then this curve is no longer its boundary because my surface lives on both sides of this curve.

youdao